Velocity type microphone



Oct. 23, 1951 H. F. oLsoN l-:TAL .2,572,376

v VELOCITY TYPE MICROPHONE v Filed May 28; 1?'548 l Y ATTORNEY Patented Oct. 23, 1951 UNITED STATES PATENT QF FICE 2,572,3161, VELOCITY TYPE MICROPHONE Harryf FL Olson,- Princeton,l and John. Prestom.

Meted'econkg. N..J.,.assignors, tio, Radio. Corporation of America', aj corporation of Delaware Appl'fcationMay 2s, 194s., sexismo. 29,814.

l, This invention relates to velocity typeI microphones, and more particularly to a high frequency equalizer for microphones of this type.

In high quality velocity type microphones, it is desirable that the response be effective over 'aswidei a range. of useful: frequencies; as; possible. I-I'eretofore, the response of conventional velocity -vmicrophonesf has had` a. tendency to'. falli oi" in the high frequency range.

The primary object* of our present; invention is to. improve the high. freuuency responsey of a velocity.v microphone by maintaining uniform. re.- sponse; throughout the useful' range of.' higlrl fre- -quencies In accordance with our present. invention, we

ay high frequency equalizer for equalizing and `accentuating the high frecul'ency response'or a velocity microphone. The. high frequency: equal'- izer of our inventionr comprises. a pair ofi perforated plates disposed in spaced parallel. relation on opposite sidesof the. Aribbon' cfa velocity type microphone. The plates: provide a. resfonanceY chamber on. each. side.v of' the ribbon and the perforations. in the plates: provide a plurality of passages for transmitting sound waves there,- through. The twov perforated plates. function as. partial reectors for sound waves.' transmitted therethrough and thereby provide. a. standing wave system between the two plates. lstandine wave system, therefore, prevents. the falling off of the difference in sound wave pressure on opposite sides of' the ribbon at the higher fre,- cuencies.v and maintains: uniform: response throughout. the useful range.` of: the microphone.

rEhe novel features of.` our invention, both as to its organization. and method'of operation; as -well as additional objects'. and advantages thereof, will best be understood from the following detailedv description when read'in connection with the accompanying drawing', which Fig. 1 is a diagram of the acoustical'. impedance characteristics of the elements. of (1)- a conventional velocity` microphone, andV (27 a velocity microphone constructedy in accordance with our invention,

Fig. 2 is a diagram of thel responsey frequency characteristics of (11). a conventional`r velocity microphone, and' (2) a velocityf microphone con*- structed in accordance with our invention,` Fig. 3 is a. diagram showing the ratio of the differentialV pressure between two perforated plates to the differential pressure in free space. as a function dh,

Q Fig. 4 isl a. View in section,v taken on the line 7 GlaimS.. (Cl. 179-1155) have provided an acoustic. means in the form of 2, 4.-4 of Fig. 5, showing, a conventional velocityV microphone constructed in accordance with our invention but. with a. portion of the. perforated plate cut away, and

Fig., 5x is.` a .sectionala side viewf of` the microphone showny in: Fig; 4; taken on thefrline, f-ir. ofi Fig. 4.

In a velocity microphone, because ofa difference in phase in the soundi wave betweeni the two sides of'the ribbon conductor there results a difference in pressure between the. two sides of the ribbon. The ribbon is driven by this difference in sound wave pressure between its two sides; The differencein pressure Ap between the two sides of the ribbon, in-a plane sound wave, where p1 and p2 are the sound wave pressures on the front and back of the ribbon, may be expressed approximately as:

Ap=p1-p2=2pm cos (Kat) sin cos 0) (l) where pm=amp1itude of the sound*v pressure,

x=wavelength,

crvelocity of sound,

t=time,

d=effective distance between. the front and back of the ribbon,y and, i

1 -angle between the normal' to the ribbon and the directionof the incidentsound.A

The volume. current of the. ribbon is giveny by I Where The acoustic impedance, aan', due; to the electri-i cal circuit, is given by the equation:y

, wir mi Aeg where B=ux density in the air gap,

Z=length of the ribbon,

A=surface area of one side of the ribbon, 2E=tota1 electrical impedance=rn1+enzl rm=e1ectrical resistance of the ribbon, and 2E2=electrical impedance of the electrical load.

The acoustic impedance characteristics of the elements of a conventional velocity microphone are shown in Fig. 1 of the drawing by solid black lines which represent the following:

:uAA=l(M1-l-M2) =the air load acoustic reactance, x A R=w (M R) =the positive acoustical reactance of the ribbon,

z :the negative acoustical reactance of .ize-

@CAR

the ribbon,

rAA=rA1+rA2=the air load acoustical resistance,

xAE=the positive acoustical reactance due to the electrical system,

x"AE=the negative acoustical reactance due to the electrical system,

AV'p/p=the ratio of the difference in pressure between i the two sides of the ribbon of a conventional Velocity l microphone to the pressure in free space, and

Ap1/p=the ratio of the difference in pressure between the two sides of the ribbon of a velocity microphone provided with a high frequency equalizer according to our present invention to the pressure in free space.

In terms of Equation 2, the characteristics depicted in Fig. 1 are as follows:

Where U=vo1ume current of the ribbon, and A=surface area of one side of the ribbon.

The voltage generated by the motion of the ribbon is given by the equation: 1

where e=generated voltage, B=flux density,

Zs-jlength of the ribbon, and

:velocity of the ribbon.

From Equations 8 and 9, Equation 10 representing the voltage generated by the ribbon, upon movement. may be written:

The voltage response frequency characteristic of a conventional velocity microphone at the terminals of the ribbon-to-line transformer is shown by the solid black line inY Fig. 2 of the drawing. It will be seen that the response falls off above 8000 cycles. This is due to the fact that the difference in pressure, Ago/p does not continue to increase linearly with the frequency.

We have found a way to counteract this loss in pressure and thereby prevent the falling oil in response at the high frequencies. This is based upon the recognition that if two parallel, perforated plates having holes or openings of predetermined size are place in a plane wave sound field, the ratio of the differential pressure Api between the plates to the differential pressure Ap in free space will increase with frequency and become a maximumwhen and then decrease to a minimum when d=%7\ and then increase again. This variation in differential pressure is due to a standing Wave system between the two plates which is created by the reflection of the sound wave between the two plates due to the fact that the acoustical impedance of the plates dilfers from the free field acoustical impedance. The ratio of Api/Ap as a function of d/k is shown in Fig. 3. The characteristic of Fig. 3 shows that the ratio'of Apr to Ap for constant incident pressure increases with frequency and becomes a maximum at d/)\=.5. This increase in differential pressure may be used to compensate for the departure of A10/1D from linearity at the high frequencies. i

In Figs. 4 and 5, there is shown a conventional microphone I which is equipped with two plates 3 and 5 each having a multiplicity of perforations 1 therein and comprising a high frequency equalizer in accordance with our present invention. The microphone includes a light, moving, electroconductive ribbon element 9 which is supported for vibratory motion in the air gap between the pole pieces of a magnetic structure I3 which provides a magnetic eld in the air gap in a manner well known in the art. The plates 3 and 5 are also supported by the supports Il in spaced, parallel relation with the ribbonQ and are preferably at least coextensive with the ribbon 9 both in length and in width. In other words, the plates 1 should be at least as long as the vibratory portion of the ribbon 9, and preferably somewhat longer so as to extend beyond the ribbon 9. The plates 'l should also be at least as wide as the ribbon 9 and, in practice, are made to extend beyond the side or longitudinal edges of the ribbon, as clearly shown in Fig. 4. In addition,the plates 'l should be spaced apart a distance of the order of one-half wave length at the frequency for which maximum response is desired. The standing wave system is also dependent upon the size of the openings 1. In this connection, it will be noted that in order to set `up a standing wave system, the acoustic impedance of the openings should be substantially less than the characteristic acoustic impedance of free space. It is obvious that, if the radius of the openings is large enough, the openings would oier very little impedance to theV sound waves, and there would be no reflection of sound waves between the plates to set up a standing wave.

Accordingly, by reducing the size of the openings and controlling the size in accordance With the frequency range desired, the standing wave set up between the plates can be controlled. The size of the holes or openings 1, and the spacing .between the plates 3, and the ribbon 9, are, therefore, a function of the frequency range to be compensated for and, therefore, may be varied according to the condition to be corrected. It will be recognized by those skilled in the art that the plates form two resonance chambers l5, I1, one on each side of the ribbon, which function to produce a standing wave system between the plates 3,- 5 for sound waves transmitted through the perforations 1, the plates reflecting between them the sound waves transmitted by them. By virtue of this standing wave system, the necessary difference in pressure is maintained throughout the frequency range to be compensated for, as shown by Figs. 1, 2 and 3 of the drawings. The difference in pressure between the two sides of the ribbon 9, with the plates 3, 5 in place, is shown by the dotted characteristic in Fig. 1. It will be seen that the pressure is also increased in the manner shown by Fig. 3. The voltage response frequency characteristic with the plates in place is shown by the dotted characteristic in Fig. 2. By employing the plates 3, 5, it will be seen that a uniform output is maintained to 15,000 cycles.

From the foregoing description, it will be apparent to those persons skilled in the art that we have provided an acoustic means for maintaining uniform response in a conventional velocity microphone throughout the useful range of high frequencies. Although we have shown and described a single embodiment of our invention, other modifications and changes are possible. Therefore, we desire that the foregoing shall be considered merely as illustrative and not as limiting.

We claim as our invention:

1. In an electro-acoustical device including means for producing a magnetic field, a vibratile element mounted for movement in said magnetic field, perforate plate means disposed in parallel, spaced relation to and on opposite sides of said vibratile element to provide a resonance chamber in which said element is immersed, and means whereby said means produces a standing wave system in said resonance chamber.

2. An electro-acoustical device according to claim 1 characterized in that said standing wave system is operative to enhance the frequency response of said device in the high frequency region, and characterized further in that said perforate plate means has perforations therein the size of which is a function of said high frequency.

3. An electro-acoustical device according to claim 1 characterized in that said vibratile element is spaced intermediate each of said plates, characterized further in that said standing wave system is operative to enhance the frequency response of said device in the high frequency region, and characterized still further in that the spacing between said vibratile element and said plates is a function of said high frequency.

4. An electro-acoutical device according to claim 3 further characterized in that said perforate plate means has perforations therein the size of which is a function of said high frequency.

5. In an electro-acoustical device including means for producing a magnetic field, a light, electroconductive, pressure-gradient responsive body mounted for movement in said magnetic field for the purpose of converting said bodys motions into electrical energy, means on opposite sides of said body in parallel spaced relation thereto providing resonance chambers on each f side of said body, said means including a pair of plates having a multiplicity of passages therethrough and being arranged to produce a standing wave system therebetween.

6. An electro-acoustical device according to claim 5, characterized in that said plate means are mounted in overlapping relation to said vibratile element.

7. In an electro-acoustical device including means for producing a magnetic field, a vibratile element mounted for movement in said magnetic field, and perforate plate means disposed in parallel, spaced relation on opposite sides of' said vibratile element to provide a resonance chamber in which said vibratile element is immersed, said plate means being spaced apart a distance of the order of 1/2 wavelength at the frequency for which maximum response is desired thereby to provide a standing wave system for said resonance chamber.

HARRY F. OLSON. JOHN PRESTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,242,964 Williams et al May 20, 1941 2,305,598 Bauer Dec. 22, 1942 2,309,109 Hathaway Jan. 26, 1943 2,346,395 Rettinger Apr. 11, 1944 

